Add information about scons share cache in build system docs

**Your Godot version:**
v4.2.dev.custom_build.b7c2fd2e9

**Issue description:**
The build time has significantly improved now for the editor. Enabling the SCONs cache improves it further for incremental builds.
The Introduction to the buildsystems document can be updated to explain how to use the SCONS cache.

I can update the docs if this is OK.

**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/stable/contributing/development/compiling/introduction_to_the_buildsystem.html#cleaning-generated-files